Bowstring Cam for Compound Bow

ABSTRACT

A bowstring cam assembly for compound archery bow has a bowstring cam member that is journaled at an end of the bow power limb. First and second power cams are affixed onto the bowstring cam member to rotate with it, and first and second flexible inextensible cam cables extend over the periphery of the associated power cams. The cam cables and the power cams cooperate to determine draw characteristics for the bow. The bowstring cam member is provided with an acircular cam profile. The second power cam is situated to one side of the axis of the bowstring cam, such that at an end of travel, i.e., at full release, both the first and second power cables are pulling in a common direction.

This application claims priority under 35 U.S.C. § 119(e) of ProvisionalPatent Application Ser. No. 60/873,238, filed Dec. 7, 2006.

BACKGROUND OF THE INVENTION

This invention is directed to the field of archery, and morespecifically to compound bows of the type employing cams and controlcables to achieve a programmed draw weight, and the latter beingvariable with draw length. The invention is more particularly concernedwith improvements to such compound bows which make the bows more compactand streamlined, and which permit the archer to select the bow's drawcharacteristic, and which increases the bow's shooting performance.

This invention is more particularly directed to compound bows of thetype that are described in my earlier U.S. Pat. No. 6,776,148 (Aug. 17,2004). That patent is incorporated herein by reference. The invention isdirected to an improvement to the bowstring cam member of the typedisclosed therein. The bowstring cam members are supported on the outerends of the upper and/or lower power limbs, and rotate to take up andrelease the bow string.

A bow of this general type is described in my earlier U.S. Pat. Nos.5,388,564 and 6,067,974. Those patents are incorporated herein byreference. Archery bows with programming means incorporated into them toregulate draw weight are also described in U.S. Pat. Nos. 3,854,417;3,923,035; 3,486,495; and 4,287,867. These bows have means to regulatetheir draw weight so that a maximum pull weight is attained at anintermediate draw position, and with the draw weight dropping to somefraction of full draw weight at the full draw position. It is also anobjective of such bows to transfer as much as possible of the energythat is stored in the bow to the arrow, so that the arrow will flyfaster and farther for a given draw weight. These goals have beendifficult to achieve.

A number of compound bows have included one or more bowstring camssupported at the outer end of the spring limb or power limb. Typicallyone eccentric cam is provided on one limb, and there is a circular wheelat the end of the other limb. In addition, the bowstring cams currentlyused all are configured so that the radius increases on draw, i.e., thedistance from the axis of the cam to the point of contact with thebowstring, i.e., the tangent with the bowstring. Accordingly, the radiusdecreases when the cam rewinds the bowstring. With that system, the rateat which the bow string moves forward becomes smaller as the bowstringapproaches the fully released, i.e., brace position. This means that thebowstring does not accelerate the arrow optimally, and at least some ofthe energy stored in the power limb is wasted. Bows that employbowstring cams are discussed, e.g., in Andrews et al. U.S. Pat. No.6,082,346 and Despart et al. U.S. Pat. No. 6,474,324. In an idealcompound bow, all the energy stored in the power limbs should betransferred to the arrow to maximize the flight of the arrow. Also, anyenergy that remains in the bow will cause bow noise. That is, theinefficient design of the prior compound bows will cause the bowstringto snap when the arrow is released. Previous proposals for compound bowsinvolving bowstring cams have not configured the mechanical advantage ofthe cam relative to the bowstring to maximize the energy transfer to thearrow.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedbowstring cam assembly for a compound bow that avoids the drawbackspresent in the bows of the prior art.

It is another object to provide a cam that increases the bow performancefor a given draw weight, and is quieter than current bows.

It is a further object to provide a bowstring cam assembly thatoptimizes draw characteristics and energy transfer of the bow.

It is still another object to improve overall behavior of the compoundbow.

One aspect of this invention involves an improvement to compound bows ofthe type that include a riser having an upper end and a lower end, withupper and lower resilient power limbs, i.e., spring limbs, that havetheir inboard ends affixed to the upper and lower ends of the riser, andsynchronizing means, e.g., a cam and cable arrangement, for ensuringequal flexing of the upper and lower power limbs upon draw and releaseof the bowstring. In the bow that employs the bowstring cam(s) accordingto preferred embodiments of the invention, upper and lower bowstringcams each are rotatably held at their axes on outboard ends of the upperand lower power limbs, and there are also associated upper and lowerpower cam arrangements, each having one or more inboard cams rotatablyheld at a rigid portion of the riser, one or more outboard power camsaffixed onto the associated bowstring cam to rotate with it, and one ormore flexible inextensible cam cables extending over the periphery ofthe associated inboard and outboard power cams. The power camarrangement is configured so as to cooperate and thus to determine drawcharacteristics of the bow. In embodiments of this invention, thebowstring cams are provided with a cam profile with a lobe or apexoriented such that the radius, from the axis thereof to the tangent withthe bowstring, diminishes as the bowstring is drawn and increases as thebowstring returns after release from a drawn position to the full braceposition. This changes the mechanical advantage of the cams on thebowstring after the bowstring is released so as to accelerate thebowstring and arrow. In other words, the rotational energy of thebowstring cam is transferred more efficiently to the arrow.

In some preferred arrangements, the bowstring cams may be forged of alightweight metal. The bowstring cams may rotate over an angle exceeding90 degrees between fully drawn and fully returned positions. The offsetratio of the bowstring cam, i.e., the size of the base circle relativeto the lobe of the cam, may be on the order of ⅔, and in one preferredembodiment about 0.633. The profile of the bowstring cam achieves anoptimal acceleration of the bow string and arrow, so that more of thebow's energy is transferred as kinetic energy to the arrow. This alsogives the bow a quieter action.

In the present improvement to the bowstring cam, which may be at theupper limb, lower limb, or both limbs of the bow, the bowstring cam isprovided with an acircular profile, and there is a second power cam onone side or on both sides of the bowstring cam. The second power cam isoffset to one side of the bowstring axis, such that at the end of travel(such as when the bowstring is fully released) the power cables arepulling in a common direction to rotate the bowstring cam. Thisconfiguration results in improved acceleration of the arrow throughoutthe travel of the bowstring.

There is a pivot pin on the bowstring cam assembly axis, and thisextends outward beyond the second power cam, and is journaled at the endof the power limb.

The offset of the second power cam is achieved by having a crank memberaffixed onto the second power cam at the side away from the first powercam and the bowstring cam, with the pivot pin then projecting out fromthe crank member.

The first and second power cam members can be located on one side onlyin some embodiments, but more favorably there are first and second powercams on both sides, for a balanced action and to permit the bow to beconfigured as a quad-cam compound bow. In that case, there are crankmembers disposed outside of the second power cams on each side.

In any case, the second power cam is situated entirely to one side ofthe pivot pin axis. The rotational travel of the bowstring cam assemblyis such that at full draw the power cables are pulling in opposeddirections, but as the bowstring cam assembly rotates back to therelease position the tangent, or pivot point, of the second power cableto the second power cam moves across the axis, so that at the end oftravel both first and second power cables are pulling in a commondirection.

The above and many other objects, features, and advantages of thisinvention will present themselves to persons skilled in this art fromthe ensuing description of preferred embodiments of this invention, asdescribed with reference to the accompanying Drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side view of the cam assembly of an embodiment of thisinvention, with the bowstring drawn back.

FIG. 2 is an edge-on view thereof.

FIG. 3 is a side view of the cam assembly, with the bowstring released,i.e., not drawn, and thus showing the cam member(s) rotated from theorientation shown in FIGS. 1 and 2.

FIG. 4 is an edge-on view showing the offset of the second power cammember relative to the pivot pin or axle.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The compound bow on which the improved bowstring cam may be employed isdescribed in my earlier U.S. Pat. No. 6,776,148, and that bow isdescribed briefly here, but the contents of that patent are incorporatedhere by reference.

The compound bow has a riser or handle portion at its center with upperand lower power limbs or spring limb portions with inboard ends that areaffixed onto at the upper and lower ends of the riser. The bow isconsidered in its normal, upright shooting orientation, as isconventional. There are upper and lower bowstring cam assemblies, e.g.,bowstring cam 20, that are pivotally or rotationally attached atrespective pivot axes to the outboard ends of the power limb members. Abow string 24 is attached to each bowstring cam 20 and rides in aperipheral groove or channel 26 in each of these cams, which groove isadapted to receive the bow string 24. Synchronizing pulleys arepivotally mounted on the riser near the ends. A continuous synchronizingcable is reeved to the synchronizing pulleys and passes over idlerwheels and through a vertical cable passage in the riser. The action ofthe synchronizing pulley and cable system is well understood, and isemployed for ensuring even flexing of the upper and lower limbs.

An inboard cam member is affixed onto the synchronizing pulley andcarries a pair of power cables, here shown as cable 30 and cable 31. Thepower cables 30, 31 may be reeved to the inboard cam(s) (not shown). Thepower cable 30 rides in a cam groove of a first outboard power cammember 32 that is mounted coaxially with the associated bowstring cam20. The second power cable 31 rides in a cam groove of a second powercam 60, which is mounted on the bowstring cam assembly, outside of thefirst power cam 32, i.e., on the side remote from the bowstring cam 20.It should be noted that the inboard cam members may be mounted on upperand lower rigid pylons that project proximally (toward the archerposition), which improves the mechanical advantage. That is, the pylonpositions the inboard cam member so that the power cables 30 and 31 pullthe outboard cams at a relatively steep angle relative to the powerlimbs, and pull against a point that is fixed in relation to the bowriser, and not against the other bow limb. The first power cable 30 iscarried in a peripheral groove or channel on the first power cam member32, passing over a lobe 48 in the cam member, and going to an attachmentor anchor point. The second power cable 31 rides in a peripheral groovein the second power cam member 60, passing over one or more lobes, andbeing attached at an anchor point. There is a fulcrum point 66 shownhere in FIGS. 1 and 3 where the power cable 31 is tangent to the secondcam member 60. The location of the fulcrum point 66 changes duringrotation.

Means are employed for preventing the limbs from becoming twisted whenthe string 24 is drawn, and this can be achieved by employing a quad camaction, as disclosed in my prior patent. Here, for each of the upper andlower limbs, there are a pair of inboard power cam members supportedcoaxially on the associated pylon, a pair of outboard cam membersmounted on either side of the associated bowstring cam 20, and two setsof cam cables 30 and 31. The cables 30 and 31 are flexible, butinextensible. A quad cam arrangement has been described in my earlierU.S. Pat. No. 6,067,974. In this compound bow arrangement, thesynchronizing wheel or pulley and synchronizing cable(s) are configuredin a manner such as is discussed in that patent. The spring limb orpower limb is formed of a pair of parallel spring components with thebowstring cam member 20 and the outboard cam members supported betweenthe ends of the two parallel spring components. A journal member ispresent at the outboard end of each spring component, and supports thecam members.

In the preferred version of this compound bow, the upper bowstring camand the lower bowstring cam would be mirror images of one another.

The bowstring cam member 20 has the groove 26 in which the bowstring 24rides on its circumference or periphery. The bow string passes over aprotuberance or cam lobe portion 42 of the cam member 20, and followsthe groove 26 around the periphery of the cam member to an anchor point44. In this embodiment, the ratio of the radius of the main base circleof the cam member 20 from the axis 22 to the groove 26, to the radiusfrom the axis to the groove at the apex of the lobe 42, may becalibrated for optimum performance. The amount of cam offset selectedmay vary from one bow to another, or from one archer to another. Theamount of cam rotation between full draw and full brace, i.e., rotarystroke, for the bowstring cam member 20 may exceed ninety degrees ofarc, and can be about 135 degrees.

The inner power cam member (not shown) may have an insert portion thatcan be interchanged with another insert portion of a different shape inorder to bring about a desired draw characteristic for the bow.

The bowstring cam member 20 in this embodiment may be molded or forged,or made from extruded metal stock, and may have cutouts and spokes tomake it light in weight. The cutouts serve to relieve some of the massof the cam member, reducing the rotational inertia without any sacrificeof rigidity. The bowstring cam member 20 positioned on the outboard endof the power limb increases its effective diameter from a full drawposition (i.e., fully rotated out) to brace height (fully returned) whenthe archer releases the bow string. This acts to increase arrow speed byaccelerating the bow string during take up. The rotational energy in thebowstring cam member 20 at the end of travel, coupled with the increasein radius at that point, causes a more efficient transfer of mechanicalenergy into the bowstring 24 and the arrow. This mechanism allows thebow to maintain bow string tension all the way from full draw to braceheight. The bow maintains string tension as well as or better than othertypes of compound bows, and especially better than those that have a camaction that is the reverse of the cam members of this invention. Thismechanism works quite effectively on the quad cam system bow, but thebowstring cam member that embodies the principles of this invention canbe used to advantage with other compound bow systems.

As shown in FIGS. 1, 2, 3, and 4 of the Drawing Figures, the improvedversion of the bowstring cam assembly has first power cams 32 and secondpower cams 60 disposed on each side of the bowstring cam member 20. Thesecond cam member 60 is disposed outside, or below, the axis 22 of thepivot pin 62 or axle of the bowstring cam assembly. The pivot pin oraxle 62 is affixed on each side to a crank portion 64 that defines anoffset from the pivot pin 62 to the inside portion of the cam 60. Inother words, there is a void behind the crank portion 64 where thesecond power cable 31 can cross the axis 22 during the rotary travel ofthe bowstring cam assembly.

Note that at the end of travel after the bowstring is released, both ofthe power cables 30 and 31 are pulling together to turn the cam member20. In the prior art, the two power cables, i.e., the two halves of thepower cable, always pull against one another.

The power cable 31 contacts the second power cam member 60 at thefulcrum point 66. Initially, i.e., before the bowstring 24 is drawnback, the fulcrum point is below or to the outside of the axis 22 of thepivot pin 62. As the bowstring is drawn back, the cam 60 rotates andallows the fulcrum point 66 to move to the other or superior side abovethe axis of the pivot pin 62, as shown in FIGS. 1 and 2. The effects thelet-off, i.e., decrease in draw weight as the bowstring 24 is drawnfully back. As rotation increases, the mechanical advantage increases asthe fulcrum point 66 moves out radially from the axis 22, opposite tothe position of the corresponding fulcrum point of the power cable 30 onthe first power cam member 32.

When the bowstring is released to shoot the arrow, mechanical advantagechanges in the opposite sense, continuously increasing the force on thebowstring as it accelerates the arrow.

At the end of travel, as shown in FIGS. 3 and 4, both ends of the powercable 30 are pulling the cam 20 in the same direction for maximum forceand speed. This results in an optimal transfer of energy from thebowstring to the arrow up until the bowstring reaches the fully releasedposition as shown in FIG. 3.

While the invention has been described and illustrated in respect to aselected preferred embodiment, it should be appreciated that theinvention is not limited only to that precise embodiment. Rather, manymodifications and variations would present themselves to those of skillin the art without departing from the scope and spirit of thisinvention, as defined in the appended claims.

1. An upper and/or lower bowstring cam assembly adapted to be rotatablyheld at an axis thereof onto a power limb of a compound archery bow, thebowstring cam assembly comprising: a bowstring cam member provided withan acircular profile and having a peripheral groove thereon adapted toreceive a bow string; first and second power cams affixed onto thebowstring cam member to rotate therewith, each of said first and secondpower cams each having a peripheral groove respectively adapted toreceive first and second flexible inextensible power cables, such thatthe cam cables and the power cams cooperate to determine drawcharacteristics for said bow; and wherein said second power cam issituated entirely to one side of the axis of the bowstring cam assembly,such that at one end of rotational travel of the bowstring cam assemblyboth said first and second power cables are situated on the same side ofsaid axis, and are pulling in a common direction to rotate the bowstringcam assembly.
 2. The bowstring cam assembly of claim 1 comprising apivot pin disposed on said axis and extending outward beyond said secondpower cam.
 3. The bowstring cam assembly of claim 1 comprising a crankmember affixed onto said second power cam, and defining an offset, and apivot pin extending out from said crank member at said axis.
 4. Thebowstring cam assembly of claim 1 wherein said first power cam isaffixed adjacent said bowstring cam member, and said second power cam isaffixed to said first power cam on a side thereof remote from thebowstring cam member.
 5. The bowstring cam assembly of claim 4comprising a crank member affixed onto said second power cam, anddefining an offset, and a pivot pin extending out from said crank memberat said axis.
 6. An upper and/or lower bowstring cam assembly adapted tobe rotatably held at an axis thereof onto a power limb of a compoundarchery bow, the bowstring cam assembly comprising: a bowstring cammember provided with an acircular profile and having a peripheral groovethereon adapted to receive a bow string; a pair of first power camsaffixed onto opposite sides of the bowstring cam member to rotatetherewith; a pair of second power cams affixed onto opposite sides ofthe bowstring cam member to rotate therewith; each of said first andsecond power cams having a peripheral groove respectively adapted toreceive first and second flexible inextensible power cables, such thatthe cam cables and the power cams cooperate to determine drawcharacteristics for said bow; and wherein said second power cams eachare situated entirely to one side of the axis of the bowstring camassembly, such that at one end of rotational travel of the bowstring camassembly both said first and second power cables are situated on thesame side of said axis, and are pulling in a common direction to rotatethe bowstring cam assembly.
 7. The bowstring cam assembly of claim 6comprising pivot pins disposed on said axis and extending outward beyondsaid second power cams.
 8. The bowstring cam assembly of claim 6comprising a pair of crank members affixed onto said second power cams,and defining an offset, and a pair of pivot pins extending out from saidcrank members at said axis.
 9. The bowstring cam assembly of claim 1wherein said first power cams are affixed adjacent said bowstring cammember, and said second power cams are affixed to said first power camson sides thereof remote from the bowstring cam member.
 10. The bowstringcam assembly of claim 4 comprising a pair of crank members affixedrespectively onto said second power cams, and defining offsets, and apair of pivot pins extending out from said crank member at said axis.